1. Field of the Invention
The present invention relates to a catalyst for selectively transforming carbon monoxide CO, which is generated as gas byproduct during production of hydrogen gas from a variety of hydrocarbon fuels such as natural gas, LPG, and kerosene, into methane CH4, a method of producing and an apparatus using such a catalyst. The technique the present invention discloses, with which hydrogen rich gas with a CO concentration of 10 ppm or less can be generated stably and also a catalyst used therefor can be produced at low cost, is suitably applicable to, for example, fuel reformers in home-use power generation systems using a polymer electrolyte fuel cell.
2. Description of the Related Art
Since polymer electrolyte fuel cells operate at low temperature of around 80 degrees C., if hydrogen rich gas serving as fuel contains carbon monoxide (hereinafter referred to as CO) at a certain level or higher, the anode platinum catalyst undergoes CO poisoning, suffering from a problem of reduction in the power generation capacity and finally making power generation impossible.
In order to avoid CO poisoning, in home-use polymer electrolyte fuel cell power generation systems using hydrogen rich gas transformed from utility gas, LP gas, kerosene, or the like by a fuel reformer, it is desirable to keep the CO concentration of gas incoming into the anode of the fuel cell constantly at 10 ppm or less. Many actual systems, in which generated gas is mixed with air in the final stage of the fuel reforming process, employ a selective CO oxidation catalyst for oxidizing CO contained in the gas to CO2.CO+½O2═CO2  (Reaction Formula 1)
Although the CO concentration of gas incoming into a selective CO oxidation catalyst is often designed to be within the range from 0.5 to 1.0%, it is difficult, with the performance of existing selective CO oxidation catalysts, to ensure that the CO concentration is 10 ppm or less throughout the life of the system. In addition, the rate of CO selection of existing catalysts is not 100% and supplied air (oxygen) is also consumed partially in the oxidation of hydrogen, suffering from an essential problem in that the CO concentration is not lowered sufficiently but hydrogen is wasted.H2+½O2═H2O  (Reaction Formula 2)
Most actual systems address the problem by providing selective CO oxidation catalyst layers in multiple stages and distributing and supplying air to each catalyst layer stage in an amount 1.5 to 3 times as large as stoichiometrically required to oxidize CO entirely, as disclosed in Japanese Patent No. 2869525 and Japanese Patent Application Laid-Open Publication No. 2001-240402, for example. This selective CO oxidation catalyst multi-staging (doubling typically) technique requires not only double amount of a selective CO oxidation catalyst containing a noble metal, but also an air supply pump, a control system therefor, and further a structure for mixing supplied air and reaction gas homogeneously in each stage, resulting in a significant increase in the cost for accessories, materials, and processing. This also leads to an increase in the size and capacity of fuel reformers.
Besides using a selective CO oxidation catalyst as above, there has also been proposed using a selective CO methanation catalyst as disclosed in Japanese Patent Application Laid-Open Publication No. Hei 3-93602, Japanese Patent Application Laid-Open Publication No. 2007-252988, and Applied Catalysis A, 326 (2007) 213-218 (Robert A. Dagle et al), for example. Further, Japanese Patent No. 3865479 discloses combining a selective CO oxidation catalyst with a selective CO methanation catalyst. Japanese Patent Application Laid-Open Publication No. 2007-203129 discloses a method of producing various high-activity and highly-durable catalysts for production of hydrogen using atmospheric-pressure plasma, but makes no mention of a selective CO methanation catalyst. Since selective CO methanation catalysts cause CO to react with H2 to be CH4, which is harmless to platinum electrode catalysts, there is no need to use a pump for supplying air externally, having the great cost advantage that the reformer can have a simple and small structure.CO+3H2═CH4+H2O  (Reaction Formula 3)
However, CO methanation reaction involves CO2 methanation reaction as a side reaction.
Since CO2 exists in hydrogen rich gas at a concentration higher than that of CO, CO2 methanation reaction would unpreferably consume large amounts of hydrogen.CO2+4H2═CH4+2H2O  (Reaction Formula 4)
Therefore, selective CO methanation catalysts are required to have a high methanation activity for CO but a low methanation activity for CO2 (i.e. have a high CO selectivity). In addition, a so-called reverse water-gas-shift reaction, in which CO2 reacts with H2 to be CO, is unignorable at high temperature and required to be suppressed.CO2+2H2═CO+2H2O  (Reaction Formula 5)
Most of previously reported selective CO methanation catalysts have a temperature range of no more than about 30 degrees C. and, at the widest, 50 degrees C., within which a high CO activity and a high CO selectivity are satisfied simultaneously. Thus, such catalysts are not necessarily stable to unexpected systemic condition changes, which may cause the outgoing CO concentration to increase and/or the temperature of catalyst layers to rise rapidly as a result of CO2 methanation reaction, an exothermic reaction, to be uncontrollable. Selective CO methanation catalysts still cannot solve these critical problems that affect the reliability of systems. There are few reports showing that selective CO methanation catalysts have been employed on practical systems, though potentially capable of significantly reducing the cost for power generation systems.
Here will be described in further detail the reason why existing selective CO methanation catalysts could not necessarily ensure the reliability of systems sufficiently. In order to reduce CO with a concentration of 1% incoming into a selective CO methanation catalyst to achieve an outgoing CO concentration of 10 ppm or less, the catalyst is required to constantly have a conversion efficiency of 99.9% or more. Even if the incoming CO concentration may be reduced by half to 0.5%, the catalyst is still required to have a high conversion efficiency of 99.8% or more. Although the catalyst itself intrinsically has a high activity, changes in operating conditions that can naturally occur in actual reaction processes, that is, even a slight fall in the temperature and/or a slight increase in the incoming CO concentration could have a substantial impact on the increase in the outgoing CO concentration under operational circumstances with such an ultimate conversion efficiency. Particularly, in the case of an operation near the lower limit of the temperature window of the catalyst, this will lead to a fatal result.
On the other hand, since CO methanation reaction is likely to proceed at higher temperature, only such a fall in the temperature and/or an increase in the incoming CO concentration as mentioned above are less likely to cause the outgoing CO concentration to increase rapidly on the high-temperature side of the temperature window. CO2 methanation reaction and reverse water-gas-shift reaction are rather major influences on the high-temperature side. A rise in the temperature of catalyst layers, if occurred for some reason, would suffer from a problem in that these two side reactions consume large amounts of H2. In addition, the heat of CO methanation reaction added with that of CO2 methanation reaction would cause a rapid rise in the temperature of catalyst layers, which may finally lead to loss of control of the reactor due to temperature runaway. This phenomenon might cause irreparable damage on the catalyst performance and the reactor.
In order to solve these practical problems, a selective CO methanation catalyst is desirable with which the activity of CO methanation reaction is improved significantly in the low-temperature range and the temperature at which CO2 methanation reaction and reverse water-gas-shift reaction, side reactions on the high-temperature side, start to occur is made further higher to dramatically expand the temperature window for stable operations.